Nylon is the generic name for a family of polyamide polymers characterized by the presence of the amide group-CONH. In the food industry, thermoplastic flexible films are used to keep food fresh prior to consumption, or for cooking food products. Greater use of centralized processing of foods in conjunction with increased handling and longer delivery times associated with long distance transportation have increased the demand for packaging films.
In the poultry and meat segments of the food industry thermoplastic flexible films are utilized to maintain freshness. Meat is frequently sold fresh, frozen or cooked; therefore films advantageously provide protection at various temperatures. Food items such as primal and subprimal cuts of beef, ground beef and processed meats are known to use coextruded or laminated films which utilize such compositions as nylon, polyester, copolymer of vinylidene chloride (PVDC), ethylene-vinyl acetate copolymer (EVA) and ionomers.
It is also generally known that selection of films for packaging food products includes consideration of such criteria as barrier properties, cost, durability, puncture resistance, flex-crack resistance, FDA approval, machinability, optical properties such as gloss and haze, printability, sealability, shrinkability, shrink force, stiffness, and strength.
Generally, nylon films are made by processes which include casting or blown film and these films may be uni- or biaxially oriented. Specific types of nylon such as nylon 6, nylon 6,6, and nylon 12 have been made into films. Known advantages of nylon films relative to other film materials in packaging applications include good oxygen and flavor barrier characteristics, durability at low temperatures and thermal stability. Certain nylon films can be used in oriented multilayer films. These multilayer films may also include one or more additional layers of films made of various resins, for example, low density polyethylene (LDPE), ethylene-vinyl acetate copolymer (EVA), ionomer, PVDC, or copolymers of ethylene and methacrylate. Nylon containing films have also been used in vacuum packaging of fresh meat. Typical and generally known films suitable for packaging and information on film manufacture are described in the Encyclopedia of Polymer Science and Engineering 2nd Ed., Vol. 7, pp. 73-127, Vol. 10, pp. 684-695 (John Wiley & Sons, Inc., 1987) whose teachings are hereby incorporated by reference.
Manufacturers and wholesalers use flexible thermoplastic packaging films to provide economical, sanitary containers, which help protect and/or preserve the freshness and wholesomeness of their products. These films are often sold in bag form. For example, a single or multilayer film is made into bags using a tubular film or one or more flat sheets or webs of film by well known processes involving e.g. cutting, folding and/or sealing the film to form bags. These films and bags may be printed and may also be uniaxially or biaxially oriented, heat shrinkable, irradiated, or may contain film layers which are abuse resistant or puncture resistant or which are crosslinked or which enhance or retard or prevent transmission of light, gases, or liquids therethrough.
In many packaging applications, it is desirable that heat shrinkable films also have good heat seal properties. Heat shrinkable bags can be made from heat sealable films. A typical food packaging bag has three sides heat sealed by the bag manufacturer leaving one open side to allow product insertion. For example, a processor may insert fresh, frozen or processed meat, ham, poultry, cheese, primal or subprimal meat cuts, ground beef, fruits, vegetables, bread or other products making a final seal to hermetically enclose the product in the bag. This final seal may follow gas evacuation (i.e. vacuum removal) or replacement of the gaseous environment within the bag by one or more gases to provide some advantage such as to assist product preservation. Food packaging bags can be made by transversely sealing tubular stock of monolayer or multilayer film and cutting off the tube portion containing the sealed end; by making two spaced apart transverse seals on tubular stock and cutting open the side of the tube; by superimposing flat sheets of film and sealing on three sides; or by folding a flat sheet and sealing two sides. This final seal is frequently a heat seal similar to the initial seals produced by the bag manufacturer although the actual heat sealing equipment may vary.
Typically, heat seals are made by applying sufficient heat and pressure to adjacent film layer surfaces for a sufficient time to cause a fusion bond between the plastic film layers. After a product is inserted, the bag is typically evacuated and the bag mouth sealed to enclose the product. Heat sealing techniques are now commonly employed to produce the final closure of the bag. For example, a bag mouth may be either hot bar sealed or impulse sealed. In making a hot bar seal, adjacent thermoplastic layers are held together by opposing bars of which at least one is heated to cause the layers to fusion bond by application of heat and pressure across the area to be sealed. For example, bags may be made from a tube stock by making one hot bar bottom seal transverse to a tubular film. Once the bottom seal is made, the tube stock can be transversely cut to form the mouth of the bag. An impulse seal is made by application of heat and pressure using opposing bars similar to the hot bar seal except that at least one of these bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name “impulse”) to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are typically cooled (e.g. by circulating coolant) while continuing to hold the bag inner surfaces together to achieve adequate sealing strength.
Advantageously, multiple packages comprising heat sealable films may be simultaneously sealed by overlapping packages and applying a sealing means, such as heat or electricity, to the appropriate portions of the packages to provide a sealed enclosed volume therein. Simultaneous sealing of multiple packages can provide advantages such as increased efficiency and throughput. However, if the exterior layer of the overlapping packages is not adequately heat resistant, simultaneous heat sealing can cause overlapping packages to become sealed to each other. Therefore, to allow sealing of overlapping multiple packages, the outermost layer of heat sealable packages and films should be designed to have enough heat resistance to not seal or adhere to another overlapping bag. For example, U.S. Pat. No. 5,480,945 to Vicik discloses nylon resin blends comprising an amorphous nylon copolymer and certain copolyamide polymers. However, the addition of heat resistant materials to allow for overlapping simultaneous sealing of multiple packages may also cause an undesirable reduction in the overall free shrink of the package films. Therefore, there is a need for polymer compositions that can be used to make single or multilayer films useful in forming food packaging having desirable levels of heat resistance with also desirable levels of heat shrink characteristics.